Just about anywhere you submit your writing will have guidelines for you to follow. Some places offer very detailed guidelines, right down to telling you how big a margin to have on each page (usually 1-inch all the way around) and what font to use (12-point Times New Roman seems to be popular). Other places have guidelines that are so vague they leave you with more questions than before.
No matter how detailed or vague, it's imperative you read the guidelines before submitting. This means that if the publisher doesn't want to see any zombie stories, don't decide your zombie thriller is just so well-written the publisher will make an exception just for you. It won't. All you'll do is waste an editor's time by submitting something that never had a chance of being accepted. Editors and agents have precious little time as it is. The guidelines are in place to help protect their time as much as possible.
If you read and follow the guidelines as closely as possibly before submitting anywhere, you'll do two things: increase you chances of getting accepted, and you'll make the editor or agent's life just a little easier by not sending something they don't accept ever.
Yes, guidelines can be just that: guidelines, not hard and fast rules. Publishers and agents might occasionally make an exception for something so well-written they fall in love with it. But 99.9% of the time, if the manuscript doesn't follow the guidelines, it will get rejected without a second thought. Don't make the mistake of assuming your manuscript is the .1% exception; odds say it isn't. I know it's hard to realize that not everyone will love your story as much as you do, but that's the way the writing world works. It's pretty subjective and what one editor hates, another might love.
The important thing to remember is to research anywhere you're thinking of submitting and read the guidelines carefully. If your manuscript doesn't meet what the publisher or agent is looking for, look somewhere else. Not only will you be saving the editor or agent from wasting time on something they'll never accept, you'll increase your chances of acceptance by submitting to the appropriate market.
Happy writing, and remember to read the guidelines!